Assign mesh to mesh vertex-group / bone
The bus will be moved by the "body" bone if all vertices are assigned to the "Body" vertex group and the bus object has an armature modifier pointing to the armature of the "body" bone.

In fact, in your file if you select a moving object (i.e. a wheel), you'll notice that the automatic procedure has inserted an armature modifier:



While if you select the bus no armature modifier is found, so the bus doesn't move even if its vertices are assigned and weighted to the "body" bone:



The simplest method to have your rig work is: go to pose mode and select the body bone; then select the bus; shift select the body bone again; Ctrl P "bone option", which skips the vertex groups and solidly parent the whole mesh to the body bone:



But a new problem arises: the bus doesn't move along with its wheels and accessories: this is because (i.e. a wheel) has a double parent so it gets double movement request: the wheel mesh is parented to the bus mesh, so - when the bus moves the wheel mesh moves; but the wheels vertices are moving too, due to the armature and bones assignements!

So you'll have to remove all parenting between wheels and bus, leaving to the armature the work to make the spare parts move together:



Last consideration: the usual" workflow would be: join all pieces in one single mesh, Shift select the armature, Ctrl P "with empty groups", select all vertices except wheels and assign them to the body bone, select all vertices of a  wheel and assign them to the wheel bone.